Electro-therapy is the application of electrical energy to the body of a human patient to provide a therapeutic effect. The therapeutic effects produced by electro-therapy include the blockage of pain, residual pain relief possibly due to the release of endorphins or other opiate-like analogs, relief from headache pain, increase of blood flow, increases in the range of motion, cartilage regrowth or regeneration, accelerated bone growth, electronic epidural for childbirth and other beneficial effects that result from the introduction of a low frequency electric field into tissue beneath the skin. Electro-therapy as defined by this application does not include electro-osmosis, electroporation, or iontophoresis, or any other process in which electrical energy such as an electrical field or electric currents are used to promote the transdermal transportation of chemicals or fluids into or out of the body. Nor does it include electrosurgery where radiofrequency electrical energy is used to cut or cauterize tissue.
Electro-therapy typically employs a non-invasive technique to introduce the electrical energy into the patient's body. Disposable electrode pads are placed on the epidermal surface of a patient and coupled to an electric generator. The generator supplies two or more oscillating or complex morphology electric currents to a patient, with respective selected electrode pads separated from one another on the patient's body with a pain site located between the electrode pads with the majority of the electric field positioned perpendicular to each skin surface on which the pads reside. The electric currents have frequencies of at least about 1 KHz and differing by as little as 1 Hz up to about 250 Hz from each other. A non-linear action of nerve fiber membranes and/or other electrochemically-active structures or fluids causes a mixing of the two independent frequency signals in a volume of tissue surrounding and beneath the pads along an axis between them to produce a therapeutic effect. The mixing yields a distribution of synthesized sum and difference frequencies among which is a therapeutic low frequency equivalent to a beat frequency of the signals.
In order to penetrate the tissue beneath the skin and provide a therapeutic effect, electrical signals applied to the body must overcome the electrical impedance of the skin. Electrical impedance is a property of the skin that limits the amount of current that can pass through the skin. The top layer of the skin, the stratum corneum, is made up of dead skin cells and contributes to the skin's high electrical impedance. Dry, intact skin can have an impedance which exceeds a hundred thousand ohms. Even carefully prepared skin, i.e., where the hair has been shaved or otherwise removed, where debridement of devitalized or contaminated tissue has been performed, and where the skin's surface has been moisturized, can still have an impedance of over one thousand ohms. A potentially large voltage would be necessary to overcome the skin impedance and drive a therapeutically useful amount of electrical current through body tissues. The relatively large amount of energy required limits the amount of time that a portable generator device powered by batteries can be used.
Additionally, electrical currents may travel across or just beneath the surface of the skin, further reducing the amount of useful current provided to body tissues. This leakage current arises from the various layers of skin, and can limit the range of frequencies that can be applied to body structures. The skin layers contribute electrical capacitance and resistive properties which act as a barrier to current flow, thus requiring a larger power source to compensate for the leakage current, further limiting battery lifetime.
Biomedical studies conducted in other unrelated fields have determined ways to reduce skin impedance. For example, one study involved the use of a silicon micro-needle array to evaluate large-molecule transportation properties of the array/skin interface (See Henry, S. et al., “Microfabricated Microneedles: A Novel Approach to Transdermal Drug Delivery,” 87 J. Pharm. Sci. 922–925 (1998)). A micro-needle array is an array of small injection needles having a limited length so that a sufficient quantity of drugs can be injected though the needles into the skin, without the accompanying pain perceived by the patient as with a standard injection needle. Volunteers described the sensation of a micro-needle array insertion as being similar to affixing a piece of tape to the skin. This study showed that the micro-needle array caused a 50-fold drop in skin resistance.
In another study, an array of silver or silver with silver chloride coated spikes were used as electrodes for electroencephalography (EEG), i.e., the measurement of electrical activity of the brain. (See Griss, P. et al., “Characterization of Micromachined Spiked Biopotential Electrodes,” 49 IEEE Trans. Biomed. Eng. 597–604 (2002)). The array was applied to the forehead of the patient to monitor EEG activity. The array was used to overcome skin resistance in order to detect the weak EEG electrical signals produced by the brain.
In addition, patents have been granted for needle arrays used in conjunction with iontophoresis and electroporation. In iontophoresis, and electric field is used to accelerate ionized molecules for addition to or removal from the body. For example, Gartstein et al. disclose in their U.S. Pat. No. 6,379,324 issued on Apr. 30, 2002 a molded or cast plastic micro-needle array in combination with an anode and cathode electrodes. Ionized drugs are accelerated into the body due to the applied electric potential. Additionally, the array uses an electric field to remove fluid from the body for analysis by a biological electrochemical sensor.
In electroporation, short pulses of high electric fields are applied to the cells causing the cell wall to transiently become porous. The applied electric field is adjusted to ensure that permanent damage to the cell wall does not result. Dev et al. disclose in their U.S. Pat. No. 6,451,002 issued on Sep. 17, 2002 a method for the treatment of tumors using an array of needles. High amplitude electrical signals are applied to the needles that cause electroporation of the tissue cells between the needles. Drugs used to treat the tumor are injected through the needles contemporaneously with the electroporation, thereby increasing their introduction into the tissue cells.
Electrosurgery is the use of electrical radio frequency energy to cut tissue and coagulate bleeding during surgery. In such a procedure, the electrical energy is delivered to the patient through a probe. The probe permits the physician to direct the electrical energy to the areas of the patient's body that she wishes to cut. In order to complete the electrical circuit, a return electrode is applied to the patient. The return electrode employs a large surface area contacting the patient to reduce the current density and prevent burning of the patient's skin at the return electrode. For example, Fleenor et al. disclose in their U.S. Pat. No. 6,544,258 issued Apr. 8, 2003 a self-regulating and self-limiting electrosurgical return electrode pad. A patient lies down on top of the pad during an electrosurgical procedure. The pad has a large surface area designed to prevent high current densities and temperature rise, thereby preventing patient trauma.
Electrode pads designed for use with medical test procedures such as electrocardiograms (ECGs) typically employ an electrical conductor, such as a lead wire, electrically connected to an electrolyte disposed within the electrode pad. For example, Cartmell et al. discloses in their U.S. Pat. No. 4,699,679 issued on Oct. 13, 1987 a disposable medical electrode pad that includes two foam sheets with electrically conductive adhesive layers on their lower surfaces. The pad further includes an electrolyte gel matrix between the foam sheets. These pads are designed for monitoring electrical signals produced by the patient, but are sometimes used to apply stimulation signals to a patent, such as in electro-therapy.
It is known in the art that applying electrical energy to the skin can reduce the impedance of the skin For example, Carim et al. discloses in their U.S. Pat. No. 6,032,060 issued on Feb. 29, 2000 directing electrical energy through a medical electrode placed on the skin of the patient in order to electrically condition the skin. The reduction in skin impedance increases the ability to monitor bioelectric signals and can reduce the amount of energy necessary for electroporation or transdermal iontophoresis.
Each of the above references provide and devices are designed for sensing electrical signals generated by the body, for delivering pharmaceuticals to the body, or for performing electrical surgery on the body. These devices disclosed by the references have physical characteristics and electrical properties which make them suitable for their intended uses; however, they are not designed for electro-therapy.